A young girl named Ymir became the first Titan in 1003 B1 after she bonded with a centipede-like creature in a body of water she fell into while fleeing soldiers.
Though becoming a Titan saved her from drowning, she was still enslaved to King Fritz, not only bearing him three daughters but also using her Titan powers to aid him on conquests.
Ymir was killed in 990 B1, so Fritz had their daughters eat her remains to gain her Titan powers, thus creating the line of Founding Titans and their ritualistic cannibalism.
The Eldians continued their reign using the power of Titans, but they were eventually brought down by civil war among holders of the nine most powerful kinds of Titans.
Aghast by the violence of his people, King Karl Fritz brought down his own empire with the help of the Tybur family, and together they invented a false origin of the Titans.
Fritz planned his own defeat in the year 743 and used Colossal Titans to build walls around the island Paradis, threatening the world with their power if they tried to break in.
Fritz erased his people’s memories and told them they were the last surviving humans so that they did not try to leave Paradis; however, the Ackermans and Hizurus were immune.
The royal family hid underground and ruled as the secretive “Reiss” family, passing the power of the Founding Titan and a peaceful ideology down through their line.
The military was established in this era, including the Survey Corps, who slay Titans outside the walls. Meanwhile, the Ackermans and Hizurus were hunted to near extinction.
The main characters of the series were born in this era, and Mikasa, the daughter of an Ackerman and a Hiruzu, became adopted by Eren Yeager’s family after her parents were killed.